The present invention is related to a nonwoven web produced from conjugate fibers. More specifically, the invention is related to a filter medium of a conjugate fiber nonwoven web.
Porous nonwoven sheet media, such as composites containing meltblown or solution sprayed microfiber webs and conventional spunbond nonwoven webs have been used in various filtration applications, e.g., coolant filtration, cutting fluid filtration, swimming pool filtration, transmission fluid filtration, room air filtration and automotive air filtration. In liquid filtration applications, especially for large volume filtration applications, e.g., coolant and cutting fluid filtration, contaminated liquid typically is pressure driven onto a horizontally placed filter medium. Consequently, the filter medium needs to be strong enough to withstand the weight of the liquid and the applied driving pressure. As such, liquid filter media need to provide high strength properties in addition to suitable levels of filter efficiency, capacity and durability.
In general, composite filter media are formed by laminating a layer of a microfiber web onto a highly porous supporting layer or between two highly porous supporting layers since the microfiber layer does not have sufficient physical strength to be self-supporting. Consequently, the production process for composite filter media requires not only different layer materials but also requires elaborate layer-forming and laminating steps, making the filter media costly. Although self-supporting single-layer microfiber filter media can be produced in order to avoid the complexity of forming composite filter media by increasing the thickness of the microfiber filter layer, the pressure drop across such thick microfiber filter media is unacceptably high, making the microfiber media unfit for filter applications, especially for high throughput filter applications. An additional disadvantage of existing microfiber filter media and laminate filter media containing microfiber webs is that they tend to exhibit weak physical properties. Consequently, these filter media are not particularly useful for large volume liquid filtration uses.
Other sheet filter media widely used in the industry are cellulosic fiber webs of thermomechanically or chemically processed pulp fibers. Cellulosic fiber media are, for example, commonly used in automotive oil and fuel filters and vacuum cleaner filters. However, cellulosic fiber filter media tend to have a limited filter efficiency and do not provide the high strength properties that are required for high pressure, large volume liquid filtration applications.
Yet another group of filter media that have been utilized in liquid filtration applications are calendered spunbonded nonwoven webs, especially polyester spunbond webs. For example, calendered polyester spunbond filter media are commercially available from Reemay, Inc. under the Reemay.RTM. trademark. Typically, spunbond filter media are formed by melt-spinning a physical blend of structural filaments and binder filaments, randomly and isotropically depositing the filaments onto a forming surface to form a nonwoven web, and then calendering the nonwoven web to activate the binder filaments to effect adhesive bonds, forming a sheet filter medium that has a relatively uniform thickness. These calendered sheet filter media exhibit good strength properties. However, the filter efficiency of these spunbond filter media is, in general, significantly lower than that of microfiber filter media. In addition, the porosity distribution on the surface of the calendered spunbond filter media tends to be non-uniform. This is because when the spun filaments are randomly deposited on the forming surface, the filament density, i.e., the number of filament strands deposited for a given area of surface, of the deposited web varies from one section to another; and when the deposited fiber web is calendered and compacted to a uniform thickness, the sections of high fiber density and low fiber density form low porosity and high porosity sections, respectively. Consequently, the calendered spunbond filter media tend to have a non-uniform porosity distribution.
There remains a need for economical filter media that provide a highly desirable combination of high filtration efficiency, capacity and high physical strength.